Serine is an important precursor of tryptophan and serotonin. Previous studies of patients with chronic fatigue syndrome (CFS) demonstrated that serine (CFSUM2) was an important urinary metabolite discriminating between CFS from control subjects, and was negatively correlated with CFS neurological symptom index and total symptom index. Serine synthesis requires both alanine and glycine as precursors, and also as a result of microbial metabolism. The aim of this study is to determine if faecal microorganisms can produce serine, and how these organisms are quantitatively related to the total gastrointestinal microbial flora in CFS patients.

Three clinical faecal coliforms (Escherichia coli, Enterobacter cloacae and Proteus mirabilis) were incubated in defined broth and their metabolites were analyzed by a gas chromatogram (GC-MS). Serine was produced by the three organisms at varying concentrations. Other amino acids including serine precursors (alaine & glycine), leucine, phenylalanine and succinic acid, an organic acid, were also detected suggesting that the contribution of microbial metabolites by enteric organisms to achieve a balanced diet may be more important as previously thought.

Faecal samples from 27 CFS patients and four age and sex matched control subjects were studied. Quantitative bacteriology was performed on all samples. Seventeen (62.9%) CFS patients had a low % distribution of E.coli ( < 80% of total aerobic faecal count). In contrast none all four control subjects had a low % distribution of E.coli ( < < 0.03).

These studies showed that the low urinary excretion of serine in CFS patients reported in previous study was associated with a disturbed gastrointestinal microbial flora. Alteration in the aerobic microbial flora, particularly the Gram negative enteric organisms, may change the exogenous supply of serine and contribute to the increased symptoms expression of patients with CFS.